The Hot Zone

Earth’s sensitivity to CO2 may be twice what climate models show

Posted by Alison Hawkes on January 14, 2011
Category : Climate Science and Scientists, Climates of the Past

Climate modeling is an inexact science, and scientists have long known that the models don’t account for everything, even though precision and accuracy is a big goal. But the limitations of climate modeling may have caused scientists to underestimate the Earth’s sensitivity to CO2 by a factor of two, according to an analysis by National Center for Atmospheric Research scientist Jeffrey Kiehl.

In a perspectives piece published in the journal Science this week, Kiehl says that the models have typically not factored in some of the long term feedback processes that determine the Earth’s temperature over the course of centuries or millennia, including ice sheet loss and processes related to vegetation and carbon cycle changes. He came to this conclusion by looking into periods of high CO2 levels in Earth’s history.

At current rates of CO2 emissions, the Earth is on a trajectory to reach between 900 to 1100 parts per million CO2 in the atmosphere by the end of the century (right now we’re at 390 ppm). The last time the Earth experienced that concentration of CO2 was 35 million years ago, a world that has sea surface temperatures more than four times higher than the today. Based on this scenario, Kiehl calculated the net radiative forcing rate — or the difference between incoming and outgoing solar radiation — and compared that to estimates from other warm periods in the Earth’s history.

He found a similar magnitude of forcing in other past warm climate periods, and with confidence in those numbers estimated that the Earth was about 29 degrees Fahrenheit warmer than modern era pre-industrial levels, some 35 million years ago. “The conclusion from this analysis,” he writes, “is that Earth’s sensitivity to CO2 radiative forcing may be much greater than that obtained from climate models.”

Once the atmosphere reaches 1000 ppm CO2, he points out that it could take tens of thousands of years to return to modern day levels. “Thus, if atmospheric CO2 reaches 1000 ppmv, then human civilization will face another world, one that the human species has never experienced in its history (over the last 2 million years).”

If CO2 concentration reaches this high level, long term feedback processes will amplify global warming beyond current modeling estimates, Kiehl says. “The human species and global ecosystems will be placed in a climate state never before experienced in their evolutionary history and at an unprecedented rate.”

That’s the thing about modern day climate change. In the past, CO2 buildup appears to have happened over longer periods of time, but nowadays the rate of change puts us into a new pace for climate change.

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